Monitoring systems for monitoring physical quantities or parameters are used in many applications such as manufacturing plants, chemical processing plants, pipe lines and telephone cables. The physical quantities monitored include fluid pressure in telephone cables. This application typically requires the use of transducers at monitoring points which may be located up to one hundred miles or so from the monitoring or central station. The wire size and the number of wires that can be economically used imposes limitations on the current and voltage that can be effectively used in the system. In this kind of application, it is desired to use a large number of transducers with a minimum of transmission lines and power requirements while realizing a high degree of accuracy and reliability in the acquisition of data representing the values of the monitored physical quantities.
The U.S. Pat. No. 4,628,315 to Douglas entitled "Addressable Transducer with Improved Address Signal processing" meets the requirements for such systems. It discloses a system that requires only two transmission lines and supports a large number of remote transducers. The transmission lines supply power to the transducers, send address signals to the transducers, and carry response signals from the transducers to a central station. The response signals are generated by a current sink oscillator which modulates transmission line current at a frequency corresponding to the measured parameter.
It has been found that an improved response circuitry can overcome limitations on the quality of the line current modulation to enhance accuracy of the transmitted information.